The present invention relates generally to thermally controlled systems for infrared and other wavelength optical windows that are exposed to extreme high and low temperatures and their production, such systems allowing their use in these extreme environments by actively cooling or heating the window, respectively, via flow passages internal to the window. In the case of window heating, that situation would typically be required to prevent icing and/or condensation since, in general, a cold window does not hinder transmission.
Infrared (IR) windows are currently unable to survive the environment experienced by hypersonic vehicles traveling above Mach 3. Window temperatures can exceed 900° C. from exposure to the high heat fluxes and temperatures associated with hypersonic flight. A typical prior art IR window 102 on a section of a hypersonic vehicle designated generally by numeral 100 employing a sensor 103 (schematically illustrated), such as an IR sensor, is shown in FIG. 1. The high-speed air 104 flows along or against the exterior surface 105 of the window, thereby generating heat which will destroy the window or render the window opaque to the wavelength of light being sensed or monitored by the sensor 103.
Optical windows and domes used in hypersonic aircraft, missile systems and spacecraft for infrared imaging or other wavelength imaging demand good mechanical stability and high optical transmission in the wavelength range of the sensor that is viewing through the window. For infrared sensors, the wavelength range is typically between 0.4 micron and 12 microns. Zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, and cadmium telluride are used in applications such as IR windows which require long wavelength infrared (LWIR) optical transmission capability. The fabrication of zinc sulfide and zinc selenide via chemical vapor deposition (CVD) routes is one possible pathway. Alternatively, IR windows can be produced by forming a green body from a population of nanoparticles, depositing a layer of ZnS powder and sintering the covered green body to produce a sintered product.
One known window assembly consists of an outer window and inner window with an intervening space which can be filled by a material characterized by high thermal insulation properties or, alternatively, a cooling fluid is circulated through an intervening space so that, the entire intervening space between the outer and inner windows is filled with either an insulating gas or a cooling fluid. There are significant drawbacks to this approach. Since the sensor must look through the cooling or insulating fluid in the intervening space, the inner window is coated with an optical coating that is substantially transparent at the visible and/or the infrared frequency portion of the electromagnet spectrum to reduce reflections because the two liquid-to-window interfaces can cause reflection or variation of the wavelength, resulting in noise in the IR signal and reduced quality. Yet another drawback of this type of assembly is that, because the interior flow cross-section created by the intervening space is simply a wide-open unsupported area, it has less of an ability to withstand a large pressure gradient between the fluid in the intervening space and the interior or exterior of the window. Also, the open flow cross-section of the intervening space means that there are no defined flow passages or flow paths in intervening space, but rather only an open cross-section. Therefore, without any such flow constraints, uniform flow across the intervening space is problematic in that fluid flow can be expected to take the shortest path from the inlet to outlet, leaving other areas with little or no flow, resulting in large temperature variations across the window. While this type of prior art proposed the coolant as a single-phase liquid or gas, flow maldistribution would have been an even greater problem if an evaporating fluid had been contemplated, because the highly variable g-forces experienced in a hypersonic vehicle or missile will further exacerbate this flow maldistribution and potential flow instabilities. This is because the inertial effects on the liquid and vapor in the coolant passage will be different.
Another known window assembly includes an inner window, an outer window, and a support subsystem between the inner and outer windows defining a plurality of infrared transparent fluid flow cooling channels for cooling the outer window without adversely affecting the optical properties of either window. Like the assembly just discussed, this second known approach also proposes to have transparent fluid flow cooling passages. We have recognized, however, that it is disadvantageous for wavelengths of interest for the sensor to be transmitted through the cooling passages as that will adversely affect the sensor data and also that opaque cooling passages are a solution to that problem.
A conceptual silicon window cooled using single-phase water has also been described by Wojciechowski et al (Internally Cooled Window for Endoatmospheric Homing, AIAA, May 1992). We have found, however, that the use of single-phase water causes undesirably lower heat transfer coefficients and requires equally undesirable large mass flow rates to maintain a uniform temperature. Furthermore, the silicon cooled window concept is only applicable to MWIR since silicon substrate absorbs in LWIR.
Zinc sulfide is a common material for LWIR and semi-active laser windows and domes. Multi-spectral zinc sulfide (ZnS), made by CVD, is also commercially available. A sintering process is believed to produce a more erosion-resistant and ultra-high density IR window. Regardless of the base material being used, or the manufacturing process used to form these windows, they must be actively cooled because of the very high thermal loads that result from the high-speed airflow over the exterior during flight. Currently available IR window materials are still unable to withstand the heat fluxes and temperatures associated with these conditions. Future missile performance improvements will be impeded without the development of advanced cooling strategies for high-speed weapon windows.
We have discovered a way to address these problems by employing an internally cooled IR window that uses two-phase flow in one or more channels having hydraulic diameters less than about 0.118 inch (3 mm) within the window to maintain both a uniform temperature, a low mass flow rate (relative to single-phase flow which results in lower pressure drops) and a low temperature while also minimizing system size. The passages are optically non-transparent to a monitored or sensed light wavelength. An inlet flow restriction or valve such as a orifice, weir (partial dam of the cross-sectional flow area), capillary tube section, reduced hydraulic diameter section or adjustable throttling valve (such as a thermostatic expansion valve (TXV) or electronic expansion valve (EXV)) is provided at the inlet of each flow passage in the IR window to act as a throttling device upstream of the evaporation section of the cooling passages which will, for two-phase evaporating coolants, initiate adiabatic nucleation (i.e., evaporation) and prevent boiling (evaporation) hysteresis. This flow restriction will also act as a flow balancing mechanism for two-phase coolant flow and even single-phase coolant flow, since the pressure drop across this restriction will substantially exceed pressure drops due to the flow down an individual passage, pressure variations due to orientation, and pressure variations due to external g-forces. For two-phase coolants, the pressure drop of the flow restriction will make perturbations in the pressure drop in the two-phase coolant passageway, i.e., perturbations potentially resulting from variations in the thermodynamic quality due to variations in heat loading applied to the window, insignificant relative to the pressure drop across this upstream throttling device. This throttling device can be located at the inlet of the IR window, at the interface between the window and the inlet flow manifold or at the exit of the inlet manifold. Such a configuration keeps the flows in balance at the design flow rates, regardless of changes in g-forces, orientation, and thermodynamic flow quality due to non-uniform heat loading on the coolant passages or the window itself. In addition, either an upstream (upstream of the IR window) control or throttling valve can be used along with the passage flow restrictions to control the superheat or quality exiting the actively cooled window. As part this discovery process, we have considered the effects of material light absorption and emission, channel emission, channel diffraction, and temperature-induced wavefront deformation. At the same time, we have discovered as well, a novel method to fabricate these windows with internal cooling passages for using two-phase flow. One of many advantages of our discoveries is that they provide significantly better performance than any previous internally cooled window that has been demonstrated in the prior art such as those which use two-phase ammonia impinging onto a porous cover and vent coolant directly in front of the window (which can significantly degrade image quality) or require bonding of the window, a stainless-steel coolant tube, and a porous cover.